The ductility (deformability) of single crystal (SX) and directionally solidified (DS) superalloys is lower than in conventionally cast (CC) parts. In regions of high multiaxiality, the low ductility of SX and DS materials is further reduced (see below).
On the other hand, the thermo-mechanical loading of turbine blades requires a certain degree of ductility (deformability) due to thermal strains and high mechanical loads.
The rupture strain εR is a material limit for describing the ductility (deformability) of the material. For a safe design, the rupture strain has to exceed the mechanical strain in the design defined by the sum of the inelastic strain εI and the elastic strain εE, as shown in FIG. 1.
The rupture strain is influenced by the multiaxiality of the material. For a uniaxial 1D state of stress (see the component 10 in FIG. 2(a)) the Poisson effect leads to a fairly high rupture strain εR1D (FIG. 2(b)). A multiaxial 3D state of stress reduces (or even prevents) the Poisson effect, i.e. the deformability of a multiaxial stress state is only obtained by the elastic volume change, FIG. 3(a). Moreover, several damage mechanisms like the growth of creep pores are significantly affected by multiaxiality so that the rupture strain εR3D in this case is substantially reduced (FIG. 3(b)).
In literature, the influence of multiaxiality on ductility is described by the stress ratio
                    r        =                              σ            H                                σ            Mises                                              (        1        )            where σH=⅓ (σ11+σ22+σ33) is the hydrostatic stress and
      σ    Mises    =                    3        2            ⁢              σ        ij        dev            ⁢              σ        ij        dev            is the von Mises stress where σijdev=σij−σHδij denotes the stress deviator. The reduction of ductility is then described by the correction factor
                                          k            ⁡                          (              r              )                                =                                    ɛ              R                              3                ⁢                                                                  ⁢                D                                                    ɛ              R                              1                ⁢                                                                  ⁢                D                                                    ,                                  ⁢        where                            (        2        )                                          k          ⁡                      (            r            )                          =                  1.65          ⁢                                          ⁢                      exp            ⁡                          (                                                -                                      3                    2                                                  ⁢                r                            )                                                          (        3        )            according to Rice and Tracey, and
                              k          ⁡                      (            r            )                          =                              sinh            ⁡                          (                                                2                  3                                ⁢                                  (                                                            n                      -                      0.5                                                              n                      +                      0.5                                                        )                                            )                                            sinh            ⁡                          (                              2                ⁢                                                                  ⁢                                  r                  ⁡                                      (                                                                  n                        -                        0.5                                                                    n                        +                        0.5                                                              )                                                              )                                                          (        4        )            according to Cocks and Ashby, with n→∞ for rigid plastic deformation. Both models predict a considerable reduction of the deformability of the material due to multiaxiality (see FIG. 4).
FIG. 5 shows a central part of a gas turbine blade 11, which comprises a root 12, a platform 13 and an airfoil 14. Three different cuts 1-3 through said central part are shown in FIG. 6 with the corresponding distribution of the stress ratio r. As can be seen from FIG. 6, the multiaxiality of thick regions in turbine blades reaches values up to r=1.6. This corresponds to a reduction of the uniaxially measured ductility down to 15% using the Rice & Tracey model and 6% using the Cocks & Ashby model, respectively (FIG. 4).
Considering that the loading of turbine blades (due to pressure and centrifugal loads and non-even temperature distributions) produces mechanical strains in the order of up to 1%, a considerable ductility of the material is required.
The document U.S. Pat. No. 5,451,142 describes a method to provide a layer/coating of a high strength polycrystalline superalloy bonded to the root of a nickelbase superalloy turbine blade. This layer is plasma sprayed onto the fir tree of the blade.
The document U.S. Pat. No. 4,921,405 teaches a single crystal turbine blade having a portion of its attachment section (fir tree) layered with a fine grained polycrystalline alloy. According to the teaching, the layering is preferably accomplished by plasma spraying of the attachment section with a superalloy and hot isostatically compacting the sprayed superalloy to minimum porosity. The resulting turbine blade should have improved life resulting from the reduced low cycle, low temperature fatigue susceptibility of, and crack growth in, the composite attachment section.
In both cases, a special coating process has to be applied during manufacturing of the blade, which requires substantial additional time and cost efforts.
U.S. Pat. No. 4,582,548 describes a single crystal casting alloy for use in a gas turbine engine. Single crystal solid blades or bars were cast and machined in the longitudinal direction. After machining they were solutioned and then pseudocoated and aged. EP 1184473 A2 discloses Nickel-base single-crystal superalloys and a method of manufacturing the same. The method is similar to the one described in U.S. Pat. No. 4,582,548, the solution heat treatment of the specimen/component and the additional heat treatment steps are done after a machining step.